fsfreeze-hook is not POSIX or Dash compatible causing weird behavior on (at least) Debian
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## Host environment
- Operating system:
Debian 12.13
- OS/kernel version:
6.1.0-43-amd64 #1 SMP PREEMPT_DYNAMIC Debian 6.1.162-1 (2026-02-08) x86_64 GNU/Linux
- Architecture:
x86_64
- QEMU flavor:
<!--qemu-system-x86_64, qemu-aarch64, qemu-img, etc.-->
- QEMU version:
7.2.22 (Debian 1:7.2+dfsg-7+deb12u18)
- QEMU command line:
`virsh domfsfreeze $ID`
## Emulated/Virtualized environment
- Operating system:
Debian forky/sid
- OS/kernel version:
6.18.15+deb14-cloud-amd64 #1 SMP PREEMPT_DYNAMIC Debian 6.18.15-1 (2026-02-27) x86_64 GNU/Linux
- Architecture:
x86_64
- qemu-guest-agent:
1:10.2.1+ds-1 amd64
## Description of problem
Using the default qemu-guest-agent package on Debian forky (trixie should have the same issues from looking at the source package) and configuring an fsfreeze-hook leads to the following error on the first run of `virsh domfsfreeze`:
```
error: Unable to freeze filesystems
error: internal error: unable to execute QEMU agent command 'guest-fsfreeze-freeze': child process has failed to execute fsfreeze hook: /etc/qemu/fsfreeze-hook: 52: Bad substitution
```
Every run after that is "fine", because `/var/log/qga-fsfreeze-hook.log` was touched and exists - which causes the Dash-incompatible behavior (in particular the erroring line 52 `STATUS=${PIPESTATUS[0]}` ) to be skipped as `USE_SYSLOG` is never set to 1.
(Aside, the `USE_SYSLOG` logic isn't particularly great on systems that default to Systemd/journald.)
On the guest, after first configuring qemu-ga.conf as follows to enable fsfreeze-hook:
```
[general]
fsfreeze-hook=/etc/qemu/fsfreeze-hook
```
Adding a simple test hook script to cause execution of the loop in line 42:
```bash
cat <<'EOF' > /etc/qemu/fsfreeze-hook.d/test.sh
#!/bin/bash
echo "$@"
EOF
chmod +x /etc/qemu/fsfreeze-hook.d/test.sh
```
(Ensuring that the log file doesn't exist: `rm /var/log/qga-fsfreeze-hook.log` )
On the host, executing `virsh domfsfreeze $ID` for the guest VM, then outputs the error above.
The issue is that the script uses `#!/bin/sh` which is linked to dash on Debian and does not support arrays/PIPESTATUS.
Output of shellcheck shows the issues in details:
```
Line 27:
touch "$LOGFILE" &>/dev/null || USE_SYSLOG=1
^-- SC3020 (warning): In POSIX sh, &> is undefined.
Line 31:
local message="$1"
^-- SC3043 (warning): In POSIX sh, 'local' is undefined.
Line 46:
log_message "Executing $file $@"
^-- SC2145 (error): Argument mixes string and array. Use * or separate argument.
Line 52:
STATUS=${PIPESTATUS[0]}
^-- SC3028 (warning): In POSIX sh, PIPESTATUS is undefined.
^-- SC3054 (warning): In POSIX sh, array references are undefined.
Line 55:
if [ $STATUS -ne 0 ]; then
^-- SC2086 (info): Double quote to prevent globbing and word splitting.
```
In my opinion the `LOGFILE/USE_SYSLOG` logic can just be cut without loss - logging can be configured in the system. rsyslog can be configured to redirect to a specific file for this case (if necessary qemu-guest-agent could contain this config). journald users likely don't want it in a separate file either, or can use a journalctl filtering service appending to file or override `StandardOutput` of the qemu-guest-agent service if needed.
Alternatively, a separate config variable for a logfile could be added instead of a hard-coded path.
## Additional information
This issue has been observed by others in the past, e.g.: https://github.com/orgs/OpenCCU/discussions/3539 who have replaced the upstream script with a custom one just using `logger` : https://github.com/OpenCCU/OpenCCU/pull/3594/changes
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